1. Field of the Invention
The present invention relates to electrical switches and, more particularly, to micro-electrical mechanical system (MEM or MEMS) switches.
2. Description of the Related Art
Various micro-electrical mechanical system switches, methods of manufacture and operation are known. See, for example, U.S. Pat. No. 5,578,976, xe2x80x9cMicro Electromechanical RF Switch, Jun J. Yao, Nov. 26, 1996; U.S. Pat. No. 6,100,477, xe2x80x9cRecessed Etch RF Micro-Electromechanical Switchxe2x80x9d, John Neal Randall, et al., Aug. 8, 2000; U.S. Pat. No. 6,127,744, xe2x80x9cMethod And Apparatus For An Improved Micro-Electrical Mechanical Switchxe2x80x9d, Robert D. Streeter, et al., Oct. 3, 2000; U.S. Pat. No. 6,160,230, xe2x80x9cMethod And Apparatus For An Improved Single Pole Double Throw Micro-Electrical Mechanical Switchxe2x80x9d, Lee A. McMillan, et al., Dec. 12, 2000; and U.S. Pat. No. 6,229,683, xe2x80x9cHigh Voltage Micromachined Electrostatic Switch,xe2x80x9d Scott Halden Goodwin-Johansson, May 8, 2001, which are all incorporated in their entireties herein by reference.
In FIG. 1, a known cantilever type MEMS switch is shown, while FIG. 2 shows a known bridge/membrane type MEMS switch. Such switches are fully described, eg, in U.S. Pat. Nos. 5,578,976 and 6,100,477, and therefore need not be further discussed.
Practical implementations of the two major types of MEMS switches, namely cantilever and bridge MEMS switches, is limited by tradeoffs between operating voltages and mechanical properties of the switch.
On the one hand, the operating voltage controlling the state of the switch is limited by the available voltages on the device, which are typically low for mobile devices. On the other hand, it is desired to have a large mechanical separation between MEMS components to reduce a parasitic load of the RF line and also to avoid a self-closing of the switch induced, eg, by high power RF signals. Additionally, in order to increase MEMS switching speed, the mechanical spring constant of the switch is preferably high, aiding a fast return to the open state by the restoring force. A high mechanical spring constant also helps to avoid stiction problems in MEMS switches. However, MEMS switches with large separations and high spring constants require a strong force to close the switch, which, in turn, require higher operating voltages.
The present invention allows MEMS switches to operate at considerably smaller voltages than the standard method of controlling the switch state with an applied DC voltage, and can be easily implemented for various MEMS switch designs. The present invention helps to improve the MEMS switch performance by allowing increased spring constants and larger separation distances between electrodes (thus, decreasing stiction and parasitic capacitance problems), while operating MEMS switches at small control voltages.
Additionally, the present invention provides a method for achieving a fast settling time when opening the switch. A problem in MEMS switches is that when the switch is opened, in returning to an open state, it undergoes a damped oscillatory motion. The fully open state is not reached until a steady state is obtained. Use of the control circuit of the present invention allows a significant reduction in the time required to reach the steady rate.
According to the invention: to operate the switch (close or open, depending on the design; but for purposes of explanation, the closed state in this invention is defined as the cantilever or membrane in a down position), instead of applying a constant voltage to the control electrodes, an oscillating voltage at a frequency corresponding (eg, equal) to the mechanical resonant frequency of the MEMS movable part (cantilever or membrane, with any and all electrodes provided thereon) is applied to the MEMS control electrodes.
Because of resonance, the amplitude of the movable part deflection will increase with each cycle of the applied AC voltage, producing deflections much larger than those obtained at DC operation. The attraction force between control electrodes is larger for smaller separation between control electrodes (roughly Fxcx9c1/d{circumflex over ( )}2). As the amplitude of deflection increases, the distance between control electrodes sweeps through a minimum. When deflection reaches an amplitude such that the attractive force is greater than the restoring spring force of the movable part (primarily, the cantilever or membrane), a DC voltage is applied to the control electrodes, and the switch closes.
Accordingly, it is an object of the present invention to provide a MEMS switch arrangement which requires smaller operating voltages than typically used.
It is an additional object of the present invention to provide a MEMS switch arrangement which reduces stiction and parasitic capacitance problems.
It is a further object of the present invention to provide a MEMS switch arrangement which permits a higher spring constant of the movable part and a larger separation distance between electrodes of the MEMS switch.
It is an additional object of the present invention to provide a MEMS switch arrangement which utilizes an AC voltage for moving the movable part.
It is an additional object of the present invention to provide a MEMS switch arrangement which utilizes both a DC voltage and an AC voltage for moving the movable part.
It is a further object of the present invention to obtain faster return to open position by introducing an external damping to the MEMS switch. A voltage proportional to the deflection velocity is applied to the control electrodes 180 degrees out of phase with respect to a downward motion of the cantilever or membrane. By doing so, the mechanical oscillation of the cantilever or membrane returning to its open position is damped, and a faster return to open position is achieved.
Further and still other objects of the present invention will become more readily apparent when the following detailed description is taken in conjunction with the accompanying drawing figures.